Your browser has javascript turned off or blocked. This will lead to some parts of our website to not work properly or at all. Turn on javascript for best performance.

The browser you are using is not supported by this website. All versions of Internet Explorer are no longer supported, either by us or Microsoft (read more here:

Please use a modern browser to fully experience our website, such as the newest versions of Edge, Chrome, Firefox or Safari etc.

Nils Ryde. Photo.

Nils Ryde


Nils Ryde. Photo.

Chemical Evolution of the Inner 2 Degrees of the Milky Way Bulge: [α/Fe] Trends and Metallicity Gradients


  • Nils Ryde
  • M. Schultheis
  • V. Grieco
  • F. Matteucci
  • R. M. Rich
  • S. Uttenthaler

Summary, in English

The structure, formation, and evolution of the Milky Way bulge is a matter of debate. Important diagnostics for discriminating between models of bulge formation and evolution include alpha-abundance trends with metallicity, and spatial abundance and metallicity gradients. Due to the severe optical extinction in the inner Bulge region, only a few detailed investigations of this region have been performed. Here we aim at investigating the inner 2 degrees of the Bulge (projected galactocentric distance of approximately 300 pc), rarely investigated before, by observing the [alpha/Fe] element trends versus metallicity, and by trying to derive the metallicity gradient in the b < 2 degrees region. [alpha/Fe] and metallicities have been determined by spectral synthesis of 2 mu m spectra of 28 M-giants in the Bulge, lying along the southern minor axis at (l, b) = (0, 0), (0, -1 degrees), and (0, -2 degrees). These were observed with the CRIRES spectrometer at the Very Large Telescope, (VLT) at high spectral resolution. Low-resolution K-band spectra, observed with the ISAAC spectrometer at the VLT, are used to determine the effective temperature of the stars. We present the first connection between the Galactic center (GC) and the Bulge using similar stars, high spectral resolution, and analysis techniques. The [alpha/Fe] trends in all our three fields show a large similarity among each other and with trends further out in the Bulge. All point to a rapid star formation episode in the Bulge. We find that there is a lack of an [alpha/Fe] gradient in the Bulge all the way into the center, suggesting a homogeneous Bulge when it comes to the enrichment process and star formation history. We find a large range of metallicities from -1.2 < [Fe/H] < +0.3, with a lower dispersion in the GC: -0.2 < [Fe/H] < +0.3. The derived metallicities of the stars in the three fields get, in the mean, progressively higher the closer to the Galactic plane they lie. We could interpret this as a continuation of the metallicity gradient established further out in the Bulge, but due to the low number of stars and possible selection effects, more data of the same sort as presented here is necessary to conclude on the inner metallicity gradient from our data alone. Our results firmly argue for the center being in the context of the Bulge rather than very distinct.


  • Lund Observatory

Publishing year





Astronomical Journal





Document type

Journal article


IOP Publishing


  • Astronomy, Astrophysics and Cosmology


  • Galaxy: bulge
  • Galaxy: stellar content
  • Galaxy: structure
  • infrared:
  • stars
  • stars: abundances
  • stars: fundamental parameters




  • VR-projektbidrag: Vintergatans ursprung - spektroskopiska undersökningar av stjärnpopulationer i Vintergatans centrum


  • ISSN: 1538-3881